docs: def bar_touched(self, bar, touch):
          """Start scrolling, and record where I started scrolling."""
          self.scrolling = True
          self._start_bar_pos_hint = get_pos_hint(bar.pos_hint, *bar.size_hint)
          self._start_touch_pos_hint = (
              touch.x / self.width,
              touch.y / self.height
          )
          self._start_bar_touch_hint = (
              self._start_touch_pos_hint[0] - self._start_bar_pos_hint[0],
              self._start_touch_pos_hint[1] - self._start_bar_pos_hint[1]
          )
          touch.grab(self)